In money-changing machines, for example, it is important to distinguish between genuine and counterfeit bills or notes as well as between normal specimens and defective ones. Thus, it is frequently desirable to return a heavily damaged note to the customer but to retain a presumably counterfeit note in the machine without giving change. Such a sorting of bills or notes is also required in banks, in order to remove defective or suspect specimens from circulation.
Systems have become known in which folding money or other substantially identical papers can be classified by comparing their thickness at certain points with a reference value. Such systems, however, can be rather easily deceived inasmuch as practical considerations limit the number of test points. Thus, for example, adhesive tape may be used to splice nonmatching bill portions together along lines remote from these test points. Such tapes could also be used to mask a gap or tear, with approximation of the standard thickness.